Great Lakes bulk carrier ships carry cargo, such as iron ore, from the U.S. Great Lakes to ports across the Atlantic Ocean, where the cargo's displacement is replaced by fresh or brackish ballast water for the return trip. Before ballast water management practices were commonplace, this ballast water was subsequently off-loaded in the Great Lakes when the ship took on its next load of cargo. Unfortunately, coastal ballast water contains many species of micro-organisms that are non-indigenous to the Great Lakes and these non-indigenous species destroy local aquatic flora and fauna. To avoid this type of harm to the environment, ballast water exchange procedures, where the coastal water in the ballast tanks is exchanged with relatively harmless mid-ocean surface seawater, have been introduced.
There are two types of ballast water exchange methods, reballasting (empty-refill) and flow-through (flushing). The empty-refill method incurs the possibility of structural damage to the ship because of the discontinuity in structural loading and in addition, may cause unacceptable variations in trim and draft due to single empty tanks, and therefore, is not widely practiced. Flow-through ballast water exchange requires pumping of mid-ocean water into the bottom of a full or partially full ballast tank, allowing the water to exit the top of the tank. Since the tanks remain full at all times, the problems associated with empty-refill are avoided. However, because of mixing between the incoming salt water and the fresh water, proper flushing of the fresh water is only accomplished after the application of multiple tank volumes of sea water. In fact, calculations show that for perfectly mixed fluids, three volume exchanges are necessary to replace 95% of the original fresh water with a 100% replacement never possible. It is desired to have a flow-through ballast water exchange arrangement that more efficiently replaces the fresh water in ballast tanks.